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Physics 53 – Lynn – Fall 2006DC CIRCUITS EXPERIMENT: Physics Notes

Read Sections 1.1-1.3 of the Physics 53 Lab Manual. The following notes supplement those sections but arenot a substitute for them.

DC or “direct current” circuits are ones in which the currents and voltages at various points in the circuitare constant as a function of time. Circuits constructed from resistors and constant voltage (or current)sources are DC circuits. By contrast, circuits involving capacitance and inductance, certain activecomponents, and time-varying voltage or current sources are definitely not DC circuits. DC circuits aresimpler to analyze. However, electric generators and motors involve alternating current and thus manyfamiliar daily examples of electricity are AC rather than DC in nature.

Voltage or “electrical potential difference” is measured in Volts. Voltage drops (or gains) are potentialdifferences between two points in a circuit. For example, consider a single component (a resistor, abattery, a meter, etc.) in a circuit. We can measure the potential difference between one side of thecomponent and the other side of the component; in this case we say the voltage is measured across thatcomponent. By contrast, there is no such thing as the “voltage through the component.” Voltagemeasurements are always made with the voltmeter in parallel with the component in question.

Current, or charge flow per unit time, is measured in Amperes. Current flows through a component in acircuit, and current measurements are made with the ammeter in series with the component in question.

Kirchhoff’s Laws were a great deal for Gustav Kirchhoff. Generations of physics students have learnedhis name, and for what, exactly? Kirchhoff’s loop rule states that the sum of voltage changes around anyclosed loop in a circuit must equal zero. This rule applies to DC circuits and to the instantaneousvoltages changes in an AC circuit. In essence it is merely a special case of conservation of energy, and ofwhat we mean by defining the concept of a “potential” in the first place. Kirchhoff’s junction rule statesthat the sum of currents into any junction must equal the sum of currents out of that junction. This rule iseven simpler – it is a statement of conservation of charge! Charge can’t be created or vanish intonothingness, so whatever charge per time flows in must also flow out if the circuit is in a steady state.

Ammeters are designed to have low resistance so as not to stop up the flow of current they are attemptingto measure. HOWEVER, “low” is a relative term. In Experiment 1 you may find that your ammeter (orsome other nominally non-resistive component) has an effective resistance that is large enough to be seenin your results. Even so, it will certainly be “low” compared to the resistances used in most real-lifecircuits; these tend to be well into the hundreds of Ohms, if not kilo-Ohms and higher.

Resistors come with values quoted in Ohms. Be careful, though! Most resistors are only qualitycontrolled to +/- 5% of these values. It is generally possible to find “precision” resistors with a toleranceof only +/- 1%, but that is about as good as it gets. The moral of the story? Don’t design a circuit thatwill only work with a 500.00-Ohm resistor, unless you want to buy five bags of a hundred and test themall until you find the right one for you.Vα

β

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Here the voltmeter is set up to measurethe potential difference between points α and β, which we can also refer to asthe voltage across R2.AHere the ammeter is set up to measurethe current through R2.